Holddown mechanism for the ramp of a dockboard

ABSTRACT

A holddown mechanism for the ramp of a dockboard. The rear edge of the ramp is hinged to the dock and a counterbalancing member is connected to the ramp and serves to counterbalance the weight of the ramp and pivot the ramp upwardly to an inclined position. The holddown mechanism, which is manually releasable, serves to restrain the counterbalancing member from raising the ramp and includes a tubular housing pivotally connected to the dock. A spring is mounted in the housing and the force of the spring acts against the end of a concentrically mounted tube and urges the tube to a retracted or telescoped position with respect to the spring and housing. A ratchet bar is mounted for sliding movement within the tube and the outer end of the ratchet bar is connected to the ramp. To prevent relative movement between the ratchet bar and the tube, a double-pawl mechanism is associated with the tube and is engageable with the teeth of the ratchet bar. The pawls are manually released by a pivotable operating handle which operates through a lost motion connection to move the pawls out of engagement with the ratchet bar teeth and enable the force of the counterbalancing member to elevate the ramp.

United States Patent Potter 1 Mar. 7, 1972 [54] HOLDDOWN MECHANISM FOR THE RAMP OF A DOCKBGARD [72] inventor: Roderick B. Potter, Milwaukee, Wis.

[73] Assignee: Kelley Company, 1110., Milwaukee, Wis.

[22] Filed: June 11, 1970 [21] Appl. No.: 42,006

Primary Examiner-Jacob L. Nackenoff Attorney-Andrus, Sceaies, Starke 8:. Sawall [57] ABSTRACT A holddown mechanism for the ramp of a dockboard. The rear edge of the ramp is hinged to the dock and a counterbaiancing member is connected to the ramp and serves to counterbalance the weight of the ramp and pivot the ramp upwardiy to an inclined position. The holddown mechanism, which is manually releasable, serves to restrain the counterbalancing member from raising the ramp and includes a tubular housing pivotally connected to the dock. A spring is mounted in the housing and the force of the spring acts against the end of a concentrically mounted tube and urges the tube to a retracted or telescoped position with respect to the spring and housing. A ratchet bar is mounted for sliding movement within the tube and the outer end of the ratchet bar is connected to the ramp. To prevent relative movement between the ratchet bar and the tube, a double-pawl mechanism is associated with the tube and is engageable with the teeth of the ratchet bar. The pawls are manually released by a pivotable operating handle which operates through a lost motion connection to move the pawls out of engagement with the ratchet bar teeth and enable the force of the counterbalancing member to elevate the ramp.

16 Claims, 8 Drawing Figures PATENTEUMR 71972 sum 1 OF 3 v FIG Y v INVENTOR.

RODERICK B; POTTER Attorneys PAIENTEBMAR H972 8,646,627

sum 2 UF 3 INVENTOR RODERICK B POTTER orneys mamgnm 71912 3,646,627

SHEET 3 [IF 3 1 ENTOR. RODERICK B. POTTER Aitorneys HOLDDOW N MECHANESM FOR THE RAMP OF A DUQOARD BACKGROUND OF INVENTION A common form of dockboard is installed in a shallow depression or pit and includes a ramp that is hinged at its rear edge to the dock and an extension lip is hinged to the front edge of the ramp. Due to the substantial weight of the ramp, a counterbalancing member is employed to bias the ramp upwardly to an inclined position while a releasable holddown mechanism interconnects the dock and the ramp and locks the ramp against upward movement. When not in use, the ramp is locked or held down by the holddown mechanism in a cross traffic position in which the ramp is generally flush with the upper surface of the dock. When a truck or other carrier is located at the clock, the operator manually releases the holddown mechanism so that the counterbalancing member will pivot the ramp upwardly to an inclined position. As a consequence of the upward pivotal movement of the ramp, a lip lifting mechanism is actuated to pivot the lip upwardly to an extended position. The operator then walks outwardly on the inclined ramp and his weight will overbalance the force of the counterbalancing member to lower the ramp and move the lip into engagement with the bed of the carrier. The conventional holddown mechanism is arranged so that the ramp can be moved downwardly without restraint, yet the holddown mechanism, when engaged, will prevent upward movement of the ramp.

SUMMARY OF THE INVENTION The present invention is directed to an improved holddown mechanism for restraining the counterbalancing means from raising the ramp of the dockboard to an inclined position. The holddown mechanism includes a tubular housing having one end pivotally connected to the supporting structure of dock and a compression spring is mounted within the housing. A tube is located concentrically within the spring, and the force of the spring acts against a cap on the end of the tube to urge the tube into a telescoped position with respect to the tubular housing.

Mounted for sliding movement within the tube is a ratchet bar and the outer end of the ratchet bar is pivotally connected to the ramp. The ratchet bar is locked with respect to the tube by a differential indexing double pawl mechanism which engages the teeth of the ratchet. To release the pawls from the ratchet bar teeth the operator pivots a handle through an operating cable and pivotal movement of the handle operates through a lost motion connection to move the pawls out of engagement with the ratchet bar teeth, and thereby enables the counterbalancing mechanism to elevate the ramp to an inclined position. Upward pivotal movement of the ramp causes the ratchet to move outwardly with respect to the tube.

When the ramp is walked downwardly by the operator, the ratchet bar will move inwardly with respect to the tube to a telescoped or retracted position, and the pawls and ratchet bar teeth are arranged so that the ratchet bar can move to the retracted position without interference from the pawls. When the lip of the dockboard engages the carrier or truck, the engagement of the pawls with the ratchet bar teeth will prevent upward movement of the ramp through the force of the counterbalancing mechanism.

The holddown mechanism of the invention incorporates a lost motion connection in that the operating handle, when in the released position, must be moved through a predetermined arc of movement before the pawls will reengage the ratchet teeth. This lost motion feature prevents accidental reengagement of the pawls with the ratchet bar teeth when the operator does not maintain nearly full angular displacement of the operating handle.

The holddown mechanism, by incorporating the differential indexing double pawl mechanism, provides double the indexing sensitivity and enables the ramp to be locited at shorter increments of travel without reducing the size of the ratchet teeth. The double-pawl system also provides added security in that the breakage of one of the pawls will not destroy the holddown capabilities so that the dockboard can continue to function with a single pawl in operation. 7

As an added advantage of this holddown mechanism, the housing, tube and ratchet are all three telescoped together which provides a shorter assembly. By providing a shorter assembly, the holddown mechanism can be connected to the supporting structure at a location closer to the front of the pit, thereby resulting in a more favorable working angle with the ramp and increasing the mechanical advantage.

Other objects and advantages will appear in the course of the following description.

The drawings illustrate the best mode presently contemplated of carrying out the invention.

In the drawings:

FIG. 1 is a perspective view of the dockboard incorporating the holddown mechanism of the invention and showing the ramp in the elevated position;

FIG. 2 is a side elevation of the dockboard showing the ramp in the upwardly inclined position.

FIG. 3 is a side elevation of the dockboard with the ramp in the cross traffic position;

FIG. 4 is a longitudinal section showing the details of the holddown mechanism with the pawls in the engaged position;

FIG. 5 is an enlarged view, similar to FIG. 4, showing the pawls in the released position;

FIG. 6 is a view taken along line 6-6 of FIG. 4;

FIG. 7 is a view taken along line 7-7 of FIG. 4; and

FIG. 8 is an enlarged vertical section of the double-pawl locking mechanism.

The drawings illustrate a loading dock 1 formed with a shallow pit or depression 2 and the dockboard 3 of the invention is installed in the pit. Dockboard 3 has a structural mounting frame 4 that is mounted in the pit with the rear portion of the frame including a series of vertical channels 5 that carry a horizontal angle 6. The upper flange of angle 6 is flush with the upper surface of dock 1.

The dockboard 3 includes a ramp 7, and the rear end of the ramp is hinged to the angle 6 of structural frame 4. Ramp 7 is composed of a generally flat tread plate 8 which is supported by a series of channel-shaped, parallel beams 9 and the forward edges of the beams are welded together by a front plate 10.

To hinge the rear edge of the ramp 7 to the angle 6 two pair of lugs 11 are welded to the angle 6 and the lugs 11 straddle a U-shaped member 12 welded to the angle 13 at the rear of the ramp. Hinge pins 14 extend through the aligned openings in the lugs 11 as well as through the central slot in the U-shaped member 12. With the slotted connection provided by the U- shaped member 12, the rear edge of the ramp can shift vertically to compensate for variations in level of the bed of the carrier truck, thereby enabling the ramp to tilt slightly in the event the bed of the carrier is at a slight angle to the horizontal.

The ramp is biased upwardly to an inclined position by a counterbalancing mechanism similar to that described in the copending US. Pat. No. 3,528,l 18. The counterbalancing mechanism includes a lever arm 15 which is welded to the rear angle 13 and the lower end of the lever arm is pivotally connected by a pin 16 to the rear end of a rod 17. Rod 17 extends through the coil spring 18 and the forward end of the rod is retained within an opening in a movable spring retainer 19 by a nut that is threaded on the end of the rod. Coil spring 18 is retained within an outer tubular sleeve 20 and the forward end of the sleeve 20 is secured to the frame 4 of the dockboard by a supporting structure 2R, while the rear end of the sleeve is similarly secured to the frame by a supporting structure 22. With this construction, the force of the spring 18 acts to urge the rod 117 forwardly and thereby pivot the lever am 15 forwardly to bias the ramp upward to the inclined position, as shown in FIGS. 1 and 2.

An extensionlip 23 is hinged to the forward edge of the ramp 7 and in the storage or cross traffic position, as illustrated in FIG. 3, the lip will assume a pendant position in which it hangs downwardly in front of the ramp. As the ramp is pivoted upwardly to an inclined position, as shown in FIGS. 1 and 2, the lip 23 will be automatically pivoted outwardly to an extended position by a lip lifting and latching mechanism, not shown, which can be similar to that described in U.S. Pat. No. 3,203,002. The lip lifting and latching mechanism in itself fonns no part of the present invention.

According to the invention, a holddown mechanism 24 locks the ramp against upward movement through action of the spring 18 and the details of the holddown mechanism are best illustrated in FIGS. 4-7. The holddown mechanism includes a tubular housing 25 and end plates 26 and 27 enclose the ends of the housing. The outer surface of the lower end plate 27 is provided with a pair of lugs 28 which are pivotally connected to a bracket 29 mounted on the structural frame 4 of the dockboard. Thus, the housing 25 is fixed to the stationary portion or frame of the dockboard although the housing can pivot with respect to the frame by virtue of the pivotal connection.

A tube 30 is mounted within the housing 25 and the outer end of the tube extends through an opening in the end plate 26. A cap 31 is welded to the inner end of the tube 30 and a compression spring 32 is located around the tube and extends between the end plate 26 and the cap 31. The force of the spring 32 acts to move the cap 31 toward the end plate 27 to thereby move the tube 30 to a retracted or telescoped position with respect to the outer housing 25.

Mounted for sliding movement within the tube 30 is a ratchet bar 33 having a series of teeth 34. The outer end of the ratchet bar is attached to a lug 35 which is pivotally connected to the undersurface of the ramp 7. Thus, the housing 25 is connected to the stationary structural frame 4 of the dockboard, while the ratchet bar 33 is connected to the ramp. The ratchet bar 33 is locked with respect to the tube 30 by a pair of pawls 36 and 37. The pawls 36 and 37 are arranged such that only one of the pawls is in full engagement with the teeth 34 of ratchet bar 33 at one time. As shown in FIG. 4, pawl 36 is in the full engagement with one of the teeth 34, while the second pawl 37, is halfway between engagement with succeeding teeth. This arrangement provides even differential indexing and enables the ratchet bar to be locked at increments equal to one-half the pitch of the teeth 34.

Pawls 36 and 37 are mounted for sliding movement within a casing 38 that is connected between spaced sideplates 39. Sideplates 39 are welded to the tube 30 and thus form an integral unit with the tube 30. The lower end of each pawl 36 and 37 is provided with a slot 40, and the nose 41 of a pawl lifter 42 extends within the aligned slots 40. Pins 43 extend across the bottom of each of the slots 40 and engagement of the nose 4] with the pins 43 will move the pawls 36 and 37 out of engagement with the teeth 34 of the ratchet 33.

The pawls 36 and 37 are urged or biased into engagement with the ratchet teeth 34 by torsion springs 44. Each torsion spring includes a coiled section 45 that is mounted around a pin 46, and a second coiled section 47 which is wound around a pin 48. Pins 46 and 48 extend through aligned openings in the sideplates 39. The free end 49 of each spring 44 bears against the bottom of the respective pawls 36 and 37, so that the spring force will urge the pawls into engagement with the ratchet teeth 34.

In addition to the pins 46 and 48, a third pin 50 extends between the sideplates 39 and is in alignment with the pins 46 and 48 and limits downward displacement of the pawls.

The pawl lifter 42 includes a central hub 51 which is mounted on a shaft 52 that extends through aligned openings in the sideplates 39. The hub 51 is connected to the shaft 52 by pin 53 that extends through slots 54 in the hub as well as through an opening in the shaft 52.

To pivot the pawl lifter 42 about the axis of shaft 52, a handle 55 is attached to the shaft and extends outwardly a substantial distance from the sideplates 39. As shown in FIG. 4, the pawl lifter 42 and pawls 36 and 37 are in the engaged or inoperative position. To bias the pawl lifter 42 to the engaged position, a torsion spring 56 is wound around a sleeve 57 on the end of shaft 52. One end 58 of the spring 56 is secured behind one of the sideplates 39, while the other bent end 59 of the spring bears against the handle 55 so that the force of the torsion spring 56 will act to urge the handle in a direction opposite to that of the arrow in FIG. 4 and maintain the paw lifter 42 in the position shown in FIG. 4. In addition to the torsion spring 56, a second torsion spring 60 is wound around a sleeve 61 mounted on the opposite end of the shaft 52. One end 62 of spring 60 is secured within a slot in the shaft 52, while the opposite end 63 of spring 60 bears against the undersurface of an ear 64 of the pawl lifter 42. The torsion spring 60 acts to maintain the pin 63 in engagement with the wall 65 of the slot 54 as will be hereinafter described.

To pivot the handle 55 and thereby release the pawls 36 and 37 from locking engagement, a cable 66 is connected to the end of the handle 55 and travels over a pair of pulleys 67 and 68 mounted on the beam 9 and ramp 7. The free end of the cable 66 extends through an opening in the tread plate 8 and is provided with a pull ring 69. The pull ring 69 is normally located within a recess 70 in the tread plate. By pulling upwardly on the pull ring 69, the handle 55 will be pivoted to the position shown in FIG. 5 to thereby move the locking pawls 36 and 37 to the released position. Release of the pawls enables the counterbalancing spring to pivot the ramp upwardly to the position shown in FIG. 2, and as the ramp pivots upwardly, the ratchet bar 33, which is connected to the ramp, will be drawn out of the tube 30.

Under normal conditions the holddown mechanism will be in the locked position in which the pawls 36 and 37 are in engagement with the ratchet teeth 34 and the ramp is in the cross traffic position as shown in FIG. 3. When it is desired to release the holddown mechanism, the operator will pull upwardly on the pull ring 69, thereby pivoting the handle 55 in the direction of the arrow in FIG. 4. As the handle 55 is pivoted, the force of the torsion spring 60 will maintain the pin 53 in engagement with the walls 65 bordering the slot 54 so that the pawl lifter 42 will be correspondingly rotated until the nose 41 of pawl lifter 42 engages the pins 43. With the nose 41 in engagement with the pins 43, there will be a load on the pin 43 which will overcome the force of torsion spring 60, so that further pivotal movement of the handle 55 will cause the pin 53 to move out of engagement with the wall 65 and move into engagement with the opposite walls 71 bordering the slot 54. When this occurs further pivotal movement of the handle 55 and pawl lifter 42 will cause the nose 41 operating against the pins 43 to move the pawls 36 and 37 out of engagement with the ratchet teeth 34. With the ratchet teeth disengaged, the force of the spring 18 acting on the ramp can pivot the ramp upwardly to an inclined position and the ratchet bar 33, being attached to the ramp will move outwardly with respect to the tube 30.

The lost motion connection provided by the pin 53 and the segmental slot 54- enables the pull ring 69 to be partially released without having an immediate reengagement of the pawls 36 and 37 with the ratchet teeth 34. In some instances the operator is unable to apply a continuous unifonn pull to the pull ring, and if the tension on the pull ring is momentarily released, causing the handle 55 to pivot toward its original position by virtue of the spring 56, this partial pivotal motion of the handle will not cause reengagement of the pawls due to the lost motion connection. Thus, the pull ring is able to move several inches due to the lost motion connection without providing partial reengagement of the pawls with the ratchet teeth. When the operator fully releases the pull ring 69, the handle 55 will be urged to this original position through the action of the spring 56 and initial pivotal movement of the handle will cause the pin 53 to move relative to the pawl lifter 42 to bring the pin into engagement with the wall 65 of the pawl lifter. Continued pivotal movement of the handle 55, will move nose 41 out of engagement with pins 43 and permit the springs 44 to return the pawls to the locked position as shown in FIG. 4.

When the operator walks down on the ramp, after releasing the pull ring 69, the weight of the operator will overbalance the force of spring 18 to thereby pivot the ramp downwardly and cause the ratchet to move inwardly of the tube 30. The pawls 36 and 37 will ride over the sloping edges of the teeth 34 to permit the ramp to be walked down. The engagement of the pawls with the ratchet teeth 34 will serve to prevent upward movement of the ramp at any elevation.

The float spring 32 of the holddown mechanism functions primarily during loading or unloading of the carrier or truck. DUring loading and unloading the height of the bed of the truck may vary. For example, during loading, as a load is moved across the ramp and onto the bed of the truck the rear springs of the truck will yield and allow the truck to descend and as it does the ramp 7 and its extension lip will move down with it due to the weight of the load moving thereacross and the ratchet bar will move inwardly of the tube 30 to a further retracted position. As the load is moved toward the front of the truck so that the weight on its rear springs becomes less, the truck springs will attempt to restore the rear end portion of the truck to its approximate previous height. As this happens the float spring 32 is compressed to allow the ramp to rise with the carrier bed. The float spring 32 also comes into play during unloading of the truck and allows the ramp to rise with the rising truck bed as the load is removed from the truck and thus holds the ramp down on the truck bed. Thus, float springs 32 cooperates with the holddown mechanism to enable the ramp to float upward with the truck bed as the truck is loaded or unloaded.

While the drawings illustrate the holddown mechanism associated with a pit-mounted dockboard, it is contemplated that the holddown mechanism can be utilized with any type of dockboard employing a counterbalancing mechanism to bias a ramp to an upward position.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. in a dockboard having a ramp member mounted on a support member and having counterbalancing means for urging the ramp to an elevated position, a holddown mechanism for locking the ramp against movement toward said elevated position, said holddown mechanism comprising a tubular housing pivotally connected to one of said members, an elongated element pivotally connected to the other of said members and having a series of abutments spaced along its length, a tube disposed within the housing and mounted for movement with respect to the housing and having resilient means disposed within the housing and arranged to bias the tube to a retracted position with respect to said housing, said elongated element mounted for movement with respect to said tube between a retracted position and an extended position, a substantial portion of the length of the elongated element being disposed within the housing when the element is in the retracted position, locking means engageable with the elongated member for locking the elongated member with respect to the tube, and release means for releasing the locking means to thereby permit said counterbalancing means to move the ramp to the elevated position with movement of said ramp to the elevated position serving to move said elongated element to said extended position.

2. The dockboard of claim 1, and including biasing means for biasing the locking means into locking engagement with the abutments on said elongated element.

3. The dockboard of claim 2, wherein said release means includes an operating member movable between a locking position and a release position, movement of said operating member from the locking position to the release position acting against the force of said biasing means to release the locking means from engagement with said abutments, said release means also including a lost motion connection arranged so that initial movement of the operating member on return from the release position through a predetermined distance toward said locking position will not release the force of said biasing means and further movement of said operating means beyond said predetermined distance toward said locking position will release the force on said biasing means to pen-nit said biasing means to move the locking means to the locked position.

4. The dockboard of claim 1, wherein said elongated element is a ratchet having a series of teeth and said locking means includes a pawl engageable with the teeth of the ratchet.

5. The dockboard of claim 4, in which the teeth of the ratchet and the pawl are arranged so that the pawl will lock the ratchet against outward movement with respect to the tube and will permit free inward movement of the ratchet with respect to the tube.

6. The dockboard of claim 1, wherein said release means comprises a lever arm connecting to said locking means, a cable having one and connected to the lever arm, pulley means disposed on the undersurface of the ramp, said cable extending over said pulley means, and handle means connected to the opposite end of the cable and located on the upper surface of the ramp, pulling upward on said handle means acting to pivot said lever arm to thereby release the locking means.

7. The docltboard of claim 6, wherein the rear edge of the ramp member is hinged to the support member and said handle mans is located adjacent said rear edge, said pulley means including a first pulley located generally beneath said handle means and including a second pulley located between the first pulley and the front edge of the ramp, said cable extending over said first and second pulleys.

8. In a dockboard having a ramp member mounted on a support member and having counterbalancing means for urging the ramp to an inclined elevated position, a holddown mechanism for locking the ramp against movement toward said elevated position, said holddown mechanism including a tubular housing pivotally connected to one of said members, an elongated element pivotally connected to the other of said members and having a series of abutments spaced along a length thereof, a second elongated element disposed within the housing and mounted for movement with respect to the housing and having an end projecting beyond the housing, resilient means disposed within the housing for biasing the second element to a retracted position with respect to said housing, said first element being mounted for movement with respect to said second element between a retracted and extended position, a pair of pawls engageable with the projections on the first element for locking the first element with respect to said second element, said pawls and the projections of said first element being arranged so that only one of the pair of pawls is in full engagement with the projections at any given position of said first element, and release means for releasing the pawls from engagement with the projections to thereby permit said counterbalancing means to move the ramp to the elevated position, movement of said ramp to the elevated position serving to move said first element to the extended position with respect to the second element.

9. The dockboard of claim 8, wherein said pawls are arranged in a side-by-side relation in the direction of movement of the ratchet.

10. in a dockboard having a ramp member mounted for movement with respect to a support member and having counterbalancing means for urging the ramp from a generally horizontal position to an elevated inclined position, a holddown mechanism for locking the ramp against movement toward said elevated position, said holddown mechanism comprising a tubular housing pivotally connected to one of said members, an elongated element pivotally connected to the other of said members and having a series of abutments spaced along the length thereof, a tube slidably mounted within the housing and having an end projecting beyond the corresponding end of the housing, spring means disposed within the housing and located around the tube and arranged to bias the tube to a retracted position with respect to the housing, said elongated element being movably disposed within the tube, said elongated element, said tube and said housing being disposed in telescoped concentric relation when the ramp is in the horizontal position, a pair of locking members supported by the tube and disposed to engage the abutments of the elongated element, engagement of the locking members with said abutments preventing outward movement of the elongated element with respect to the tube, but permitting inward movement of the elongated element with respect to the tube, an actuating member carried by the tube and disposed to move the locking members between a locked and a released position, and operating means connected to the actuating member for manually operating said actuating member.

11. The dockboard of claim 10, wherein said operating means comprises a lever arm mounted for pivotal movement with respect to the tube between a locking position and a release position and connected to said actuating member, and a cable connected to the lever arm and extending to the upper surface of the ramp in position where it can be actuated by an operator.

12. The dockboard of claim 11, and including resilient means for urging the locking members into locking engagement with said abutments, movement of said lever arm from the locking position to the release position acting against the force of said resilient means to thereby release the locking members from engagement with said abutment, and a lost motion connection interconnecting the lever arm and the actuating member and arranged so that initial movement of the lever arm on return from the release position through a predetermined distance toward the locking position will not release the force of said resilient means, whereby partial loss of displacement on said cable will notmove the locking members into partial engagement with the abutments, and further movement of said lever arm beyond said predetermined distance toward the locking position will release the force of said resilient means to permit said resilient means to bias the locking members into engagement with said abutments.

13. The dockboard of claim 12, wherein said lost motion connection includes a shaft secured to the lever arm, and a projection extending outwardly from said shaft, said actuating member being provided with a recess to receive the projection with the recess being substantially larger than the projection, whereby the projection can be moved within the boundaries of the recess without a corresponding movement of said actuating member.

14. The dockboard of claim 13, and including second resilient means to return the lever arm to the locking position.

15. A dockboard, comprising a supporting structure; a ramp having its rear end hinged to the supporting structure; counterbalancing means for urging the ramp to an upwardly inclined position; and a holddown mechanism for locking the ramp against movement toward said inclined position, said holddown mechanism comprising a tubular housing pivotally connected to the supporting structure, a tube movably disposed within the housing and having an end projecting beyond the upper end of the housing, spring means disposed within the housing and arranged to urge the tube to a retracted position with respect to said housing, a ratchet bar movably mounted within the tube and having a series of teeth disposed along its length, a pawl connected to the tube and disposed to engage the ratchet bar teeth to thereby prevent movement of the ratchet with respect to the tube, a pawl actuating member operatively connected to said pawl for moving said pawl between a locked and a released position, operating means connected to the pawl actuating member and including an operating member disposed on the upper surface of the ramp in position tobe engaged by an operator and movable between a locking position an a released position, means responsive to a predetermined initial amount of movement of said operating member in a direction from the released position toward said locking position for preventing actuation of said pawl actuating member, and means responsive to movement of said operating member beyond said initial amount for actuating said pawl actuating member to thereby move the pawl into engagement with the ratchet teeth and lock the ratchet with respect to the tube.

16. The dockboard of claim 15, and including a pair of pawls, said pawls being arranged whereby only one of said pawls is in full locking engagement with the teeth of the ratchet at any position of said ratchet, and means for urging the pawls to the locked position. 

1. In a dockboard having a ramp member mounted on a support member and having counterbalancing means for urging the ramp to an elevated position, a holddown mechanism for locking the ramp against movement toward said elevated position, said holddown mechanism comprising a tubular housing pivotally connected to one of said members, an elongated element pivotally connected to the other of said members and having a series of abutments spaced along its length, a tube disposed within the housing and mounted for movement with respect to the housing and having resilient means disposed within the housing and arranged to bias the tube to a retracted position with respect to said housing, said elongated element mounted for movement with respect to said tube between a retracted position and an extended position, a substantial portion of the length of the elongated element being disposed within the housing when the element is in the retracted position, locking means engageable with the elongated member for locking the elongated member with respect to the tube, and release means for releasing the locking means to thereby permit said counterbalancing means to move the ramp to the elevated position with movement of said ramp to the elevated position serving to move said elongated element to said extended position.
 2. The dockboard of claim 1, and including biasing means for biasing the locking means into locking engagement with the abutments on said elongated element.
 3. The dockboard of claim 2, wherein said release means includes an operating member movable between a locking position and a release position, movement of said operating member from the locking position to the release position acting against the force of said biasing means to release the locking means from engagement with said abutments, said release means also including a lost motion connection arranged so that initial movement of the operating member on return from the release position through a predetermined distance toward said locking position will not release the force of said biasing means and further movement of said operating means beyond said predetermined distance toward said locking position will release the force on said biasing means to permit said biasing means to move the locking means to the locked position.
 4. The dockboard of claim 1, wherein said elongated element is a ratchet having a series of teeth and said locking means includes a pawl engageable with the teeth of the ratchet.
 5. The dockboard of claim 4, in which the teeth of the ratchet and the pawl are arranged so that the pawl will lock the ratchet against outward movement with respect to the tube and will permit free inward movement of the ratchet with respect to the tube.
 6. The dockboard of claim 1, wherein said release means comprises a lever arm connecting to said locking means, a cable having one end connected to the lever arm, pulley means disposed on the undersurface of the ramp, said cable extending over said pulley means, and handle means connected to the opposite end of the cable and located on the upper surface of the ramp, pulling upward on said handle means acting to pivot said lever arm to thereby release the locking means.
 7. The dockboard of claim 6, wherein the rear edge of the ramp member is hinged to the support member and said handle mans is located adjacent said rear edge, said pulley means including a first pulley located generally beneath said handle means and including a second pulley locaTed between the first pulley and the front edge of the ramp, said cable extending over said first and second pulleys.
 8. In a dockboard having a ramp member mounted on a support member and having counterbalancing means for urging the ramp to an inclined elevated position, a holddown mechanism for locking the ramp against movement toward said elevated position, said holddown mechanism including a tubular housing pivotally connected to one of said members, an elongated element pivotally connected to the other of said members and having a series of abutments spaced along a length thereof, a second elongated element disposed within the housing and mounted for movement with respect to the housing and having an end projecting beyond the housing, resilient means disposed within the housing for biasing the second element to a retracted position with respect to said housing, said first element being mounted for movement with respect to said second element between a retracted and extended position, a pair of pawls engageable with the projections on the first element for locking the first element with respect to said second element, said pawls and the projections of said first element being arranged so that only one of the pair of pawls is in full engagement with the projections at any given position of said first element, and release means for releasing the pawls from engagement with the projections to thereby permit said counterbalancing means to move the ramp to the elevated position, movement of said ramp to the elevated position serving to move said first element to the extended position with respect to the second element.
 9. The dockboard of claim 8, wherein said pawls are arranged in a side-by-side relation in the direction of movement of the ratchet.
 10. In a dockboard having a ramp member mounted for movement with respect to a support member and having counterbalancing means for urging the ramp from a generally horizontal position to an elevated inclined position, a holddown mechanism for locking the ramp against movement toward said elevated position, said holddown mechanism comprising a tubular housing pivotally connected to one of said members, an elongated element pivotally connected to the other of said members and having a series of abutments spaced along the length thereof, a tube slidably mounted within the housing and having an end projecting beyond the corresponding end of the housing, spring means disposed within the housing and located around the tube and arranged to bias the tube to a retracted position with respect to the housing, said elongated element being movably disposed within the tube, said elongated element, said tube and said housing being disposed in telescoped concentric relation when the ramp is in the horizontal position, a pair of locking members supported by the tube and disposed to engage the abutments of the elongated element, engagement of the locking members with said abutments preventing outward movement of the elongated element with respect to the tube, but permitting inward movement of the elongated element with respect to the tube, an actuating member carried by the tube and disposed to move the locking members between a locked and a released position, and operating means connected to the actuating member for manually operating said actuating member.
 11. The dockboard of claim 10, wherein said operating means comprises a lever arm mounted for pivotal movement with respect to the tube between a locking position and a release position and connected to said actuating member, and a cable connected to the lever arm and extending to the upper surface of the ramp in position where it can be actuated by an operator.
 12. The dockboard of claim 11, and including resilient means for urging the locking members into locking engagement with said abutments, movement of said lever arm from the locking position to the release position acting against the force of said resilient means to thereby release the locking members from enGagement with said abutment, and a lost motion connection interconnecting the lever arm and the actuating member and arranged so that initial movement of the lever arm on return from the release position through a predetermined distance toward the locking position will not release the force of said resilient means, whereby partial loss of displacement on said cable will not move the locking members into partial engagement with the abutments, and further movement of said lever arm beyond said predetermined distance toward the locking position will release the force of said resilient means to permit said resilient means to bias the locking members into engagement with said abutments.
 13. The dockboard of claim 12, wherein said lost motion connection includes a shaft secured to the lever arm, and a projection extending outwardly from said shaft, said actuating member being provided with a recess to receive the projection with the recess being substantially larger than the projection, whereby the projection can be moved within the boundaries of the recess without a corresponding movement of said actuating member.
 14. The dockboard of claim 13, and including second resilient means to return the lever arm to the locking position.
 15. A dockboard, comprising a supporting structure; a ramp having its rear end hinged to the supporting structure; counterbalancing means for urging the ramp to an upwardly inclined position; and a holddown mechanism for locking the ramp against movement toward said inclined position, said holddown mechanism comprising a tubular housing pivotally connected to the supporting structure, a tube movably disposed within the housing and having an end projecting beyond the upper end of the housing, spring means disposed within the housing and arranged to urge the tube to a retracted position with respect to said housing, a ratchet bar movably mounted within the tube and having a series of teeth disposed along its length, a pawl connected to the tube and disposed to engage the ratchet bar teeth to thereby prevent movement of the ratchet with respect to the tube, a pawl actuating member operatively connected to said pawl for moving said pawl between a locked and a released position, operating means connected to the pawl actuating member and including an operating member disposed on the upper surface of the ramp in position to be engaged by an operator and movable between a locking position and a released position, means responsive to a predetermined initial amount of movement of said operating member in a direction from the released position toward said locking position for preventing actuation of said pawl actuating member, and means responsive to movement of said operating member beyond said initial amount for actuating said pawl actuating member to thereby move the pawl into engagement with the ratchet teeth and lock the ratchet with respect to the tube.
 16. The dockboard of claim 15, and including a pair of pawls, said pawls being arranged whereby only one of said pawls is in full locking engagement with the teeth of the ratchet at any position of said ratchet, and means for urging the pawls to the locked position. 